A bleb initiation model for chemotaxis suggests a role for myosin II clusters and cortex rupture

Abstract

Blebs, pressure driven protrusions of the plasma membrane, facilitate the movement of cells such as the soil amoeba Dictyostelium discoideum and other eukaryotes such as white blood cells and cancer cells. Blebs initiate or nucleate when proteins connecting the membrane to the cortex detach, either as a result of a rupture of the cortex or as a direct consequence of a build up in hydrostatic pressure. While linker detachment resulting from excess hydrostatic pressure is well understood, the mechanism by which cells rupture their cortex in locations of bleb formation is not so clear. Consequently, existing predictive models of bleb site selection do not account for it. To resolve this, we propose a model for bleb initiation which combines the geometric forces on the cell cortex/membrane complex with the underlying activity of actin binding proteins. In our model gaps, resulting from a rupture of the cortex, form at locations of high membrane energy where an accumulation of myosin II helps to weaken the cortex. We validate this model in part through a membrane energy functional which combines stresses on the cell boundary from membrane tension, curvature, membrane-cortex linker tension with hydrostatic pressure. Application of this functional to microscopy images of chemotaxing Dictyostelium discoideum cells identifies bleb nucleation sites at the highest energy locations 96.7% of the time. Sensitivity analysis of the model components points to membrane tension and hydrostatic pressure, all of which are regulated by myosin II, as critical to model predictability. Furthermore, microscopy reveals discrete clusters of myosin II along the leading edge of the cell, with blebs emerging from 80% of these sites. Together, our findings suggest a critical role for myosin II in bleb initiation through the formation of gaps and provides a predictive mathematical model for quantitative studies of blebbing. Author summaryEukaryotic cells such as white blood cells and cancer cells have been observed to move by making spherical herniation of their plasma membrane, referred to as blebs. The precise mechanism by which cells select locations around their boundary to initiate blebs is unclear. We hypothesize that blebs initiate at locations of high membrane energy where an accumulation of myosin II helps to rupture the cortex and/or detach linker proteins. We test this hypothesis by formulating a free energy functional representation of membrane energy to predict where blebs will initiate. The functional March 16, 2020 1/19 accounts for geometric forces due to membrane tension, curvature and membrane-cortex linker tension as well as hydrostatic pressure. Application of the functional to data from the soil amoeba, Dictyostelium disodium, identifies blebs at the highest energy locations over 90% of the time. Sensitivity analysis of model components points to membrane tension and hydrostatic pressure, all influenced by myosin II, as major forces driving bleb initiation. Additionally, we observe clusters of myosin II at locations of bleb ...

Motor neuron boutons remodel through membrane blebbing

“…This hypothesis is in agreement with blebs being favored in tissues where cells encounter increased mechanical resistance 69 . In this scenario, as it has been proposed for cells in compressive 3-D environments 70 , it is possible that a muscle derived signal, released in regions of higher activity, can polarize the recruitment of molecules required to or that facilitate bouton formation to spots of high membrane tension.…”

“…While the first model is adequate when cells are subjected to high compression forces, it does not perform as well when compression force is lower 70 . Our data suggests that, although at the NMJ the two mechanisms may coexist, with elevated activity boutons tend to form mainly as a consequence of compressive forces exerted by the muscle.…”

“…While the molecular identity of the signal that leads to bouton initiation remains to be elucidated, it is well established that blebs nucleate when a small patch of membrane is detached from the actin cortex 70 . This can happen either as a direct consequence of buildup in hydrostatic pressure (that detaches membrane to cortex binding proteins) or by formation of local gaps, resulting from rupture of the cortex in regions of high membrane energy (where accumulation of myosin-II helps to weaken the cortex) 70 . While the first model is adequate when cells are subjected to high compression forces, it does not perform as well when compression force is lower 70 .…”

“…This can happen either as a direct consequence of buildup in hydrostatic pressure (that detaches membrane to cortex binding proteins) or by formation of local gaps, resulting from rupture of the cortex in regions of high membrane energy (where accumulation of myosin-II helps to weaken the cortex) 70 . While the first model is adequate when cells are subjected to high compression forces, it does not perform as well when compression force is lower 70 . Our data suggests that, although at the NMJ the two mechanisms may coexist, with elevated activity boutons tend to form mainly as a consequence of compressive forces exerted by the muscle.…”

Blebology: principles of bleb-based migration

“…However certain migratory cells like melanoma cells or zebrafish mesodermal progenitors especially under confinement show polarized myosin distribution at the cell rear leading to unequal distribution of contractility. This suggests that local pressure increase is not always essential for polarized bleb formation [20,21,49,58]. Taken together, asymmetric ERM activity and contractility are two major mechanisms to polarize blebs at the leading edge.…”